In the Hall-Heroult process, aluminum is extracted by electrolyzing aluminum oxide (also known as "alumina") dissolved in a molten salt bath based on an electrolyte containing primarily sodium fluoride and aluminum fluoride. The molten electrolyte is generally maintained at a temperature in the range of about 950.degree. C.-1000.degree. C. The electrolytic cell includes a crucible having a carbon lining which serves as the cathode, and anodes, typically carbon, immersed in the molten salt bath.
The molten salt-aluminum oxide serves as an electrolyte solution. Heat produced by the large electric current in the cell melts the cryolite which dissolves the aluminum oxide and maintains the electrolyzed aluminum in a molten state in which it collects at the cell bottom.
Although the Hall-Heroult process has been commercially used for more than 100 years, it has limitations, such as the requirement that the process operate at high temperatures on the order of about 970.degree. C. Such high operating temperatures are needed to maintain alumina solubility. However, the high temperatures also result in reactions between the carbon cathode and the cryolite and molten aluminum, thereby creating problems of metal and electrolyte containment.
Various attempts have been made in the prior art to improve carbon cathodes by providing them with adherent protective coatings of refractory materials. These protective coatings generally comprise an aluminum-wettable refractory hard metal, such as titanium diboride, alone or in combination with other materials such as metal alloys, intermetallic compounds, cermets and oxides.
Some prior patents disclosing attempts at providing carbon electrodes with an aluminum-wettable coating are Foster et al U.S. Pat. No. 4,308,115; Sane et al U.S. Pat. No. 4,560,448; Boxall et al U.S. Pat. Nos. 4,466,996 and 4,624,766; Sekhar et al U.S. Pat. No. 5,310,476 and Juric et al U.S. Pat. No. 5,352,338. However, these prior art coatings each suffer from one or more serious limitations making them less than entirely suitable for their intended purpose. For example, the particularly preferred coating described in Sekhar et al U.S. Pat. No. 5,310,476 contains silicon and phosphorus compounds. Silicon and phosphorus are generally considered deleterious to efficient and economical operation of an aluminum electrolysis cell.
It is an objective of the present invention to provide a coating composition which will produce a metal-wettable coating on carbon electrodes, without including any silicon or phosphorus compounds in the composition.
A related objective of the invention is to provide a coating composition made from relatively inexpensive ingredients.
Additional objectives and advantages of my invention will become apparent to persons skilled in the art from the following detailed description.